It was supposed to be the cellphone safety study that settled the scary question once and for all: Do mobile devices cause brain cancer?

But the most exhaustive investigation undertaken into the risks of getting tumours from using the phones has had inconclusive results. After studying the issue for 10 years and looking at brain cancer incidence in 13 countries, including Canada, a group of the world's top epidemiologists has reported paradoxical findings.

First the bad news. Heavy cellphone use, defined as chatting on mobiles for more than half an hour a day over 10 years, was associated with a 40-per-cent increase in risk of a rare and often deadly brain cancer known as glioma, the same type that caused the death last year of U.S. Senator Edward Kennedy.

But the study, known as Interphone and organized the United Nations' authoritative International Agency for Research on Cancer (IARC), found that low and moderate amounts of cellphone use seemed to offer modest protection against developing the disease.

Experts say the unusual findings, released Monday in the International Journal of Epidemiology, mean that the definitive answer on cellphone safety has yet to be given.

The question "as to whether mobile phone use increases risk for brain cancers remains open," concluded Rodolfo Saracci of Italy's National Research Council and Jonathan Samet, of the University of Southern California's department of preventative medicine in a separate commentary in the journal on the results.

The observations by the two prominent epidemiologists, experts in incidence rate of diseases, suggest that the debate over cellphone safety is unlikely to go away anytime soon.

Researchers have been investigating the cancer hazard from phones because using them amounts to placing a small radio transmitter next to the head, exposing the brain and ears to microwave radiation.

The mobile communications industry maintains that there is no cause for concern because emission levels are well within government safety standards. It says the new study confirms this view.

"When you look at the study … the conclusions are that there is no overall increased risk," says Bernard Lord, president of the Canadian Wireless Telecommunications Association, an industry trade group.

The new study was designed in the late 1990s and is only now being released, in part because the researchers weren't quite sure what to make of the results, which were based on cellphone use among more than 5,100 people diagnosed with brain cancers from 2000 to 2005. Besides Canadians, it included people from Japan, Germany, France and Israel.

Because the study has been so long in the making, its results don't reflect the recent explosion in mobile usage, a factor that worries some of the researchers on the project.

"I think these results are of concern because the study subjects were light users compared to today," commented Elisabeth Cardis, the study's lead author and a professor at the Centre for Research in Environmental Epidemiology in Barcelona.

While Dr. Cardis acknowledged the research isn't conclusive, reducing exposure to cellphone radiation "might be a reasonable course of action until stronger conclusions can be drawn around the risks." Dr. Cardis, a Canadian who headed IARC's radiation group during much of the period of the study, is one of the first prominent epidemiologists to suggest caution when it comes to cellphones.

But IARC concluded that the study didn't confirm a link to brain cancer, although the finding of increased risk among heavy users and the rapid growth in cellphone usage since the research began indicates more investigation is warranted.

"An increased risk of brain cancer is not established from the data," said Christopher Wild, IARC's director, in a statement "However, observations at the highest level of cumulative call time and the changing patterns of mobile phone use since the period studied by Interphone, particularly in young people, mean that further investigation of mobile phone use and brain cancer risk is merited."

The study $25-million, with about one-quarter of the funding from the mobile industry, including the Canadian wireless association. Cellphone companies had no role in designing the research or in vetting its results.

Dr. Cardis said the unusual results - some cellphone use seems to protect against cancer while high use seems to be associated with cancer - may be a result of the difficulty researchers had in recruiting controls. These are people without cancer who were used in the study as a comparison group to see how they differed in their cellphone usage from those with the tumours.

According to spot checks by the researchers, people volunteering to be controls used cellphones more than the population at large, suggesting the results could be skewed toward underestimating the risks from the mobile devices.

Many of the people in the high use category wouldn't be considered particularly heavy chatters by today's standards. Most people reported a lifetime total of only 100 hours. The heaviest users were classified as having a lifetime tally of more than 1,640 hours on the phones.

The average Canadian user is on a mobile 6.7 hours a month and would enter the high risk group after about 20 years. There are approximately 23 million wireless subscribers in the country or 70 per cent of the population, according to the Canadian Wireless Telecommunications Association.

Besides glioma, the researchers also tracked meningioma, a slow-growing brain tumor that is often benign. Heavy users also had a 15-per-cent elevated risk of contracting it, compared with people who don't use cellphones.

Based on the reasoning that other cancers in the head and neck may be caused by exposures to cellphones, the scientists also looked at the incidence of salivary gland tumours and acoustic neuroma, a benign tumour of the inner ear. The studies on them are to be released later.

Health Canada is reviewing the Interphone study, but said it considers mobile phones to be safe. "At this time, the department has no scientific reason to consider the use of cellphones to be dangerous to the health of the Canadians," it said.

How to limit exposure

What can those worried about cellphone safety do?

The easiest thing is to use land lines whenever possible or limit cellphones to emergency situations. Hands-free mode is another option to ensure the phone isn't near the head, as is sending text messages instead of chatting.

Some experts worry about childhood exposure because long-term usage may be a cancer risk. Given that the technology is here to stay, children will pile up many hours of usage over their lifetimes. "For children, I would certainly advise" not using them, says Louis Slesin, editor of Microwave News, a publication that tracks the hazards of electromagnetic radiation.

Mr. Slesin has some other tips: Use cellphones outside because they emit less energy when they have easier connections to the towers that relay signals. For the same reason, use in trains or cars causes more radiation exposure because the phones are constantly switching signals to new towers, an activity that leads to temporary boosts in signal power.

Cellphone manufacturers could display on packaging the amount of radiation their devices give off. However, the industry objects because this type of reporting practice suggests there may be harm from high-emitting models. The industry's position is that as long as a device meets government standards, it is safe.

The Environmental Working Group, a U.S.-based advocacy group, has tested cellphones and posted the results on the Internet. Models differ markedly in radiation output, with some high-emitting devices releasing three to six times more radiation than the lowest emitting phones, according to EWG data.

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